1. Field of the Invention
This invention relates to a method for removal of plugs from tubes and more specifically to a method for removing plugs from a heat exchanger within a nuclear reactor steam generator.
2. Background Information
A heat exchanger within a pressurized water nuclear reactor steam generator consists of a vessel enclosing a plurality of tubes, through which water heated by the nuclear reaction flows. Secondary water in the vessel surrounds the outside of the tubes such that the heat exchange occurs between the two fluids. Water in the vessel is converted to steam which powers a turbine. Do to operating stresses, tubes can crack or fracture and, if not plugged, result in the commingling of the two fluids. Accordingly, tubes are regularly inspected and any tube that appears degraded is plugged so that the radioactive fluid does not contaminate the water used to generate steam.
A common device used to plug a heat exchange tube in a nuclear steam generator is a mechanical plug as shown in U.S. Pat. No. 4,390,042 to H. D. Kucherer et al. This type of plug comprises a cylindrical shell member closed at one end and having a tapered inner bore. An expander member is disposed within the cylindrical member such that when the expander member is drawn along the longitudinal axis of the cylindrical member, the outer surface of the cylindrical member is forced into contact with the inner surface of the heat exchange tube due to the taper of the inner bore of the cylindrical shell. When installed, the plug presses against the tube with a pressure of more than 10,000 p.s.i. As such, when removal of the plug is required, simply pulling the plug from the tube is not practical.
The heat exchanger of a nuclear steam generator contains a plurality of heat exchange tubes so that the generator may operate with some plugged tubes. However, during scheduled maintenance repair of certain tubes can be effected. The plug must be removed so that the tube may be repaired and returned to service. Alternatively, some tubes will simply be returned to service. However, prior to returning the tube to service, the plug must be removed. Because the force required to simply pull the plug from the tube could damage the tube, there are several methods of reducing the pressure applied by the plug prior to plug removal. For example, as disclosed in U.S. Pat. No. 4,800,637 to Overbay, the plug may be stretched by a mechanical means to attempt to reduce its diameter and, therefore, reduce the pressure exerted by the plug on the tube. The mechanical means disclosed is a threaded rod which, when inserted into the plug, contacts the closed end of the plug where pressure is applied causing the plug to stretch. Once the plug is stretched sufficiently, the plug is pulled from the tube. This method is inefficient as both pushing and pulling forces are involved. Additionally, plugs will often break prior to being stretched sufficiently to be withdrawn, leaving a portion of the plug wedged in the tube.
In U.S. Pat. No. 5,255,717, an apparatus has been disclosed which uses arc welding to create weld beads on the interior surface of a plug. The heating and contraction of the material to form the weld beads is intended to reduce the diameter of the plug so that less force is required to pull the plug from the heat exchange tube. Following the welding, the plug is pulled out by mechanical means. This method has proved to be time consuming and often does not result in a reduced plug diameter. Additionally, because the shell has been weakened, the plug will tear leaving a portion of the plug wedged in the heat exchange tube.
As disclosed in Hawkins, U.S. Pat. No. 4,734,972, another method of plug removal involves drilling a small hole through the top of the plug, tapping the hole, core drilling the plug shell, inserting a threaded rod into the tapped hole, and removing the plug top. This procedure is not time effective because of the necessity to use two drill bits.
Finally, there is a method whereby two drill bits are used to used to remove the shell. The first drill bit has a smaller diameter than the second drill bit. As with the device disclosed in Hawkins, U.S. Pat. No. 4,734,972, this procedure is not time effective because of the necessity of using two drill bits.
Accordingly, there is a need for a plug removal method which will ensure the removal of the entire plug without tearing.
There is a further need for a plug removal method which is time efficient.
There is a further need for a plug removal method which is compatible with plugs which are presently used in nuclear reactor steam generator heat exchange tubes.